(a) Field of the Invention
The present invention relates to a die attach adhesive composition for bonding semiconductor chips to lead frames, and more particularly, to a die attach adhesive composition that has superior modulus characteristics, bonding power, adhesion strength and workability, thereby enabling use with semiconductor chips in a wide range of sizes.
(b) Description of the Prior Art
Typically, semiconductor packages comprise lead frames/substrates in which semiconductor chips are bonded onto lead frame paddles via adhesives. FIG. 1 shows a thin quad flat package (TQFP) 1, which is a type of surface-mounting semiconductor, in which a semiconductor chip 2 is mounted on a semiconductor chip paddle 3a of a lead frame 3, by an adhesive layer 6. A bond pad (no reference numeral given) on the mounted semiconductor chip 2 and internal leads 3b are electrically connected by conductive wires 4. The semiconductor chip 2 and the conductive wires 4 are protected from the exterior by a molded encapsulating resin compound 5, and external leads 3c are soldered on, for example, a circuit board, the external leads 3c serving as external connection terminals.
In the above, die attach adhesive compositions are used in bonding the semiconductor chip 2 on the paddle 3a of the lead frame 3. Here, it is necessary for such adhesive compositions to have exceptional physical and chemical properties after the composition is cured, as well as provide a sufficient level of workability. Necessary physical properties for the die attach adhesive compositions include a low coefficient of thermal expansion, high adhesive strength, high thermal- and electroconductivity, and durability, while the required chemical properties include high chemical resistance and low impurity content. With regard to the workability of the adhesive compositions, it is necessary for the same to have a suitable level of viscosity and rate of cure.
Die attach adhesive compositions meeting the above requirements and typically used include an epoxy base resin, which may have substituents; a curing agent, containing an amine compound; a diluent, such as epoxide or butyl carbitol acetate; an adhesion promoter, such as silane or dicyandiamide; a thixotropic agent, such as colloidal magnesium silicate; and other additive agents, such as silicone. Here, depending on the required characteristics, intended use, etc., modifications can be made to the epoxy resin adhesive composition to meet varying demands.
Particularly, Ag flakes are commonly used as a filler to provide the die attach adhesive compositions with superior thermal and/or electrical conductivity. Because of good dispersion in organic resin systems, silver has the advantage of easily controlling the composition's thixotropy needed upon dispersion or die placement. However, silver does not provide sufficient adhesive strength between the semiconductor chip and the chip paddle of the lead frame, thereby reducing package reliability. For example, delamination can occur at the interface between the semiconductor chip and the chip paddle and/or cracks in the package may develop.
The above is particularly true in `hard` modified die attach compositions that are high in modulus. In contrast, such problems do not occur in `soft` modified compositions. However, when a wire bonding process (a high temperature process of 240.degree. C.), subsequent to the semiconductor chip-mounting process, is performed by a bonding machine, minute vibrations are generated causing defects in the wire bonding, which the `soft` composition cannot resist.
In more detail, when the capillary of the bonding machine is pressure-welded into the outside edge of the surface of the semiconductor chip, the adhesive layer beneath the semiconductor chip is slightly distorted by the pressure of the capillary load. When the capillary is released from the semiconductor chip, damped vibration occurs, causing defects in the wire bonding or a tilt in the mounting of the semiconductor chip slant. Accordingly, `soft` modified adhesive compositions are unsuitable for the use in packages on which small semiconductor chips are mounted since the minute vibrations cannot be absorbed.
As with `hard` modified epoxy resin compositions, `intermediate` modified epoxy resin compositions, which are in modulus between `hard` and `soft` epoxy resin compositions, do not provide sufficient adhesive strength. As a result, the problems of interfacial delamination and/or cracking remain.